Multilayer optical memories of two general types have been proposed.
Pohl, Stacked Optical Memories, Applied Optics February 1974 discusses a memory comprising a stack of holographic plates on which information is written or recorded and read or accessed by time-gated signals.
Multilayer optical memories which are recorded or accessed by focused laser beams are described in each of the following:
Katayama et al 5,187,703 PA1 Best et al. 5,255,262.
Swainson et al. 4,288,861 proposes a volume or three-dimensional storage medium using multiphoton absorption effects.